I'm probably going to do this for myself, but if people are interested I will put it on my website.

I'm planning on writing a program that will remain dormant until a certain time then wake up and start playing subliminal mp3s. I will have it say things like "You are dreaming", "Do a reality check", "You are asleep"...

Since I planned to use it for myself I was going to make it hard coded, but if people are interested I'll make a GUI for it so that you can select the time that you want it to kick on (when you start your REM sleep) and what messages to play and maybe a feature to record your own messages.